Sound perception is mediated through a specialized sensory epithelium made up of mechanosensory hair cells and supporting cells. The proper development and maintenance of this epithelium is critical for the ability to hear, and loss of these cells leads to deafness. While many vertebrates, including birds and reptiles, are able to regenerate damaged hair cells, mammals (including humans) do not retain this ability, resulting in permanent deafness after injury. Gene therapy is a potentially promising therapeutic approach that might one day allow for the recovery of hearing in deaf individuals. However, before gene therapy can target gene expression, the molecular mechanisms that underlie the generation of these sensory cells needs to be understood. The long term goal of this study is to uncover the molecular mechanisms regulating the initiation of hair cell differentiation in the developing mammalian cochlea. Our lab has found that Hey1 and Hey2 are expressed during cochlea differentiation, overlapping with the onset of hair cell differentiation and rapidly down-regulated as differentiation occurs. Not only that, but our results indicate that these two transcriptional repressors are under the control of Shh signaling pathway. However, lack of Shh signaling does not seem enough to trigger hair cell differentiation. Our preliminary results suggest that Activin A, present in the cochlea sensory epithelia before differentiation, is involve in triggering hair cell differentiation. Briefly, in aim1 we will characterize the timing of differentiation of hair cells in the Hey1Hey2 double knockout, in aim2 we will determine if loss of Hey2 or Hey1 and Hey2 diminishes Shh ability to delay hair cell differentiation, and, finally, in aim3 we will study the effects of different inhibitors of the Activin A pathway in vitro and determine the effects that loss of Activin A function has in hair cell differentiation. In this stuy we will use well-established genetic tools, including mouse transgenics and cochlea explant cultures, to study the effects of disrupting these pathways on hair cell differentiation and maturation. It is our hope that the experiments put forth in this application will advance our understanding of the genetic mechanism underlying the development and maintenance of sensory cell in the cochlea.